One arrangement for changing speeds in such a motor is described and shown in the specification of French published patent application No. FR 2 658 962A and U.S. Pat. No. 5,134,531 which is incorporated herein by reference. It enables a speed of rotation of the motor at a level i, where i is in the range 1 to n-1 inclusive, to be obtained by connecting a group of i+1 brushes of the motor to the terminals of the current supply source. Reference to that specification is invited for detailed information of the principle of the commutation control means and the various advantages that result from its use in practice. Its main advantages are its relatively low price and its reduced size.
Although such a commutation control means is satisfactory in operation, it is found that, in use, it does have the drawback of giving rise to irregular wear as between the various brushes that supply the poles of the inductive winding of the motor.
Brushes used in direct current motors are generally of the electro-graphitic type, made from amorphous carbon which is converted to graphite by high temperature heat treatment and consolidated with bonding agents. The wear to which these brushes are selected is partly mechanical, being linked to the general question of sliding friction of the brushes, but it is also partly electrical in nature.
Experiments carried out by the Applicant (i.e. the Applicant for the patent application in France on which this present application is based) have shown that wear that occurs for electrical reasons is in fact predominant over mechanical wear, and that this results in wear taking place more rapidly as a function of the extent to which each brush is conducting current. For this reason, the useful life of the direct current motor of the kind mentioned above can be substantially reduced if, as tends to happen, a single brush wears out prematurely.